I purchased runequest 6 a couple of years after it came out, on the recommendation that it would be good for a Hyborage Age game that I was planning. I bought Mythras immediately upon release based on my experience with RQ6. Since that time, it has become my go to system for a large number of games.
Mythras is in the d100 family of games, and is derived from the Mongoose RQ2 rules (by the same authors actually). Having seen these rules since, I can see how they have evolved to make them more understandable, more elegant, and more in line with a game that might work for multiple settings. I would call it medium-crunch, with most of that crunch being up front at character generation. Skills are percentile rated, as it is a member of the d100 family. I like this because it is very clear what your chances are - 30% ina skill means you have a 30% chance of success.
Mythras uses the standard STR/CON/DEX/SIZ/INT/POW/CHA setup for stats, and all skills are based on combinations of these two, rather than static values seen in some other d100 games. Skills are broken into two categories - Standard skills which everyone get some base value in, and Professional Skills, which you may not have any points in.
Culture and Career define how you get your skills - each provides a set with points to distribute among them. Careers cover a broad range of medieval professions (though see Mythras Imperative for more modern ones), and Cultures cover Barbarian, Civilized, Nomadic, and Primitive. Each has a distinct flavor to them, and there are careers recommended for each. Family history and family events are also covered, providing for a more lifelike character. I find that this makes characters make sense within a society, and gives them a sense of belonging that doesn't come in every other game.
Skills can be utilized in a number of ways, but somewhat unique to Mythras is the notion of a differential roll. You compare levels of success (Critical, Normal, Failure, and Fumble) to each other and gain a number of bonuses based on this. This is used extensively in combat, but some guidance is provided on making use of it in other scenarios.
An economics and equipment section is presented as well, complete with standards of living and rules for crafting, bartering, and haggling, as well as all of the weapons and armor you might need for an ancient to medieval campaign. Siege weapons and vehicles round out the mix. One notable part of weapons is their use of Size and Reach. Combat (detailed below) makes use of this extensively. It allows for Spear and Shield users to have distinct advantages over Hand Axe and Short sword users.
The game provides an extensive section on rules for a variety of situations. Fatigue is one of the most notable here, as it is used for bleeding and asphyxiation, and heightens the tension in combat. Many of the other standard things that you expect to find in many more detailed RPGs - encumbrance, survival, falling, poisons, etc. can all be found here.
One notable call out are Passions. Passions are a measurement of loyalty, hate, love, or other emotional bonds to people, places or things. They are rated the same as skills, and indeed, in the right situations, may be used as skills. Thus, if defending your king, and you have a great Love (King), you may augment your combat skill with your Love (King) passion, or even replace it if the situation warrants. These are highly recommended in bringing your character some nuance that may not be immediately present. These can also be added to a character at pretty much any time (pending GM approval of course) so that you can grow your character quite easily.
One of the most engaging features of Mythras to me is the combat system. Mythras uses hit locations and hit points, as well as action points to determine number of actions and initiative to determine when you act. Attacking is active, as well as defense if you desire. Damage is done and armor reduces the damage. Defenses can be via parrying or evading. Abstracted, it doesn’t work on a grid, instead relying on engagement between melee combatants and movement ranges.
As noted above, differential rolls are used extensively here, and power the use of Special Effects. These reflect disadvantageous situations that the attacker or the defender may inflict upon their foe, making combat much more lively than a simple back and forth of attacks and defense. These can be Tripping your foe, Pressing your Advantage, Impaling with your spear, and more. They are not limited to attackers, as defenders can defend, even if the attacker misses, gaining their own special effects like Overextending their Opponent, returning a Trip, or Blinding with a shield. Special effects go a long ways towards making the combat more tense, interesting, and providing opportunities other than whittling down bags of hit points.
I should note, however, that Special effects can take a bit to get used to, and this is probably my strongest criticism of the game. There are quite a few options, and the novice player can become overwhelmed, or focus on simple effects like striking the head. I would recommend taking a look at Mythras Imperative with its slightly smaller list of special effects to ease into things. Or limit your players to some specific ones until they get the feel of things. Soon they will see how other options can be better. One of our first games, I suggested that the GM strike at another player using Trip as a special effect instead of striking at the head. The head shot would not have killed the other player, but the trip ended up delaying him long enough that my character did die.
Mythras has an extensive magic section, clocking in at 80 pages and covering 5 distinct magic systems - Folk Magic to cover small spells the common people might use, Animism that deals with the negotiation with and binding of spirits, Mysticism which brings a high-flying wuxia feel, highly flexible Sorcery for covering (rather intuitively) sword and sorcery campaigns, and Theism, which brings a decidedly different view of magic bestowed by deities upon their worshipers.
Each one of these is presented in extensive detail, and you are encouraged to tune and refine these to suit your campaign. Guidelines for low, medium, and high magic campaigns are provided, as well as different ways to recover magic points - not all need apply in your campaign, and may apply at different levels. Advancement is covered in detail, of course, and each can be tuned - folk magic may be quick, and sorcery may be slow to learn. Mythras’ magic systems are one of the gems of the book, in my opinion.
Rounding out the book are three sections - one for cults and brotherhoods, an extensive creature bestiary, and a games mastery section.
The cults and brotherhoods provides an excellent framework for building organizations for your campaign, including ranks within them. Many examples are given as to using them with the above mentioned magical schools, and including Gifts - special powers like Immortality or puissance in a skill - that may be learned as one gains ranks within the organization. Geasa, Taboos, Superstitions, and Oaths cover a variety of restrictions, making it so one might need to swear allegiance to a Lich-lord to gain knowledge of raising the dead.
The bestiary provides rules for creating many new monsters, as well as covering more than 50 different mythical and non-mythical creatures. Some are less traditional like the Bagini or the Acephali, and traditional goblins, giants and dragons can be found here as well. Some non-traditional PC races like minotaurs and panthotaurs can be found here, as well as more traditional elves, dwarves, and halflings.
Another real gem in Mythras is the Games Mastery section. Unlike many of these section in other RPGs, I feel Mythras’ really gives excellent guidance of how to run a Mythras game. How to use passions, how to run social conflict, how to do investigations and use of traps. The combat section here talks extensively about some of the common misperceptions of the system - options other than death, action points being the only mechanism for winning, and how to use rabble (mooks) and underlings to get a particular feel. It really talks extensively about how to give cults flavor and life, and more than just faceless organizations that the PCs ignore.